DC energy stores can be implemented from many different technologies such as batteries (e.g. lithium and sodium sulphur type), capacitors including supercapactitors and ultracapacitors, and flow cells (e.g. vanadium redox flow cells). In a typical dc energy store system an AC/DC power converter is interposed between the dc energy store and an ac power supply. More particularly, the AC/DC power converter has ac terminals connected to the ac power supply and dc terminals connected to the dc energy store by means of a dc link. The AC/DC power converter can be of any suitable type with a conventional two- or three-level topology with a series of semiconductor power switching devices fully controlled and regulated using a pulse width modulation strategy.
AC power is supplied from the ac power supply and stored in the dc energy store. In this mode of operation the AC/DC power converter acts as an active rectifier and converts the ac power to dc power that is compatible with the dc energy store. When the stored energy is needed then the AC/DC power converter can act as an inverter to convert the dc power to ac power that is compatible with the ac power supply. The AC/DC power converter is therefore capable of bidirectional power flow.
This ability to store and release energy is useful in a number of different applications including storing excess wind energy and releasing that energy at a later time when less wind energy is available. A different application is to store energy in case an electrical generator stops operating. The energy stored in the dc energy store can then be used to keep essential systems and services operating until a standby or back-up generator is started and brought into operation. In some circumstances it can also be helpful to store energy slowly and allow the energy stored in the dc energy store to be released very quickly for applications that need high power pulses of energy like the linear electrical motors that are used to start roller coasters.
The dc energy store system will normally include one or more auxiliary units such as pumps, fans, control units or other devices that are connected to the ac power supply. A control unit can be used to control the operation of the AC/DC power converter and may be associated with its own uninterruptible power supply (UPS) so that it can continue to operate during short duration transients of the ac power supply. However, the UPS can be expensive if it is designed to supply power to the control unit for long periods of time such as 10 minutes or more. If the ac power supply is interrupted or experiences voltage or frequency transients then the other auxiliary units will not receive power and this can cause problems with the practical operation of the dc energy store. For example, if the fans that are used to cool the dc energy store stop working then this can result in unacceptable temperature increases. Certain types of dc energy store also require pumps to continue running at their optimum efficiency.